1. Field of the Invention
The present invention relates to a method of preventing the occurrence of blankened margins on photographic sheets used in electrophotographic apparatus such as electrophotographic lithoplate-making machines, electrophotocopying machines and the like in which an image of a document is formed by an electrophotographic method on a photosensitive sheet.
2. Description of the Prior Art
As is well known in, for example, electrophotographic lithoplate-making machines, a photosensitive sheet with at least an electrophotosensitive material layer formed thereon is used as a master paper. The photosensitive sheet is, after being charged with electricity by a charger, transported to and placed in exposure position for being exposed to image-forming light. The regions of the photosensitive sheet exposed to the image-forming light are discharged, so that an electrostatic latent image is formed on the photosensitive sheet. The photosensitive sheet on which the electrostatic latent image has been formed is then developed and fixed in a well known manner. As a result of this, the regions exposed to no image-forming light permit toner particles to adhere thereto so as to form a visible image corresponding to the pattern of the electrostatic latent image. The remaining regions, exposed to the image-forming light do not permit toner particles to adhere thereon. The photosensitive sheet is finished in such a way that, by use of an etcher, the part with toner particles adhered is caused to be oleophilic and the remaining part to which no toner particles adhered is rendered hydrophilic. The finished photosensitive sheet is later used as a printing plate, which is fastened around the rotary cylinder of a printing machine for printing with oil inks.
Either when a photosensitive sheet having a length longer than a regular one is used or when a photosensitive sheet is placed in a different position from the specified position in order to form an image offset to one side of the photosensitive sheet, the photosensitive sheet is obliged to include parts, especially margins, where no image-forming light shines. This leads to the occurrence of blackened margins on the photosensitive sheet, which is very undesirable.
For preventing the occurrence of blackened margins, heretofore, an eraser device having a lamp unit has been used and alternatively, a lens system having a large angle of view has been used so as to shine on a widened exposure area.
A problem associated with conventional electrophotographic apparatus is that in the former the provision of a special eraser device increases the production cost of the apparatus.
Another problem associated with conventional apparatus is that in the latter alternative, the widened exposure area requires scaling up of the machinery.